The present invention relates to a method of producing 1,3-propanediol by means of the hydration of acrolein in the presence of a chelate-forming ion exchange resin followed by subsequent catalytic hydrogenation of the 3-hydroxypropionaldehyde formed thereby.
It is known in the art that 1,3-propanediol has many possibilities of use as a monomeric structural element for making polyesters and polyurethanes and as the initial substance for the synthesis of cyclic compounds.
As is known from U.S. Pat. No. 2,434,110, acrolein can be hydrated in the presence of an acidic catalyst with the formation of 3-hydroxypropionaldehyde. The reaction preferably takes place at an elevated temperature using a solution of 5 to 30% by weight acrolein in water and an acid such as for example, sulfuric acid, phosphoric acid or acid salts of these acids as catalyst. The reaction mixture obtained during the hydration is optionally hydrogenated after removal of non-reacted acrolein in the presence of customary hydrogenation catalysts. Catalysts containing one or several metals which are effective in hydrogenation such as e.g. Fe, Co, Ni, Cu, Ag, Mo, W, V, Cr, Rh, Pd, Os, Ir, Pt are suitable for the hydrogenation of 3-hydroxypropionaldehyde to form 1,3-propanediol.
The low yields of 1,3-propanediol, which can be traced in particular to side reactions consuming acrolein during the hydration stage, are a disadvantage in the method of U.S. Pat. No. 2,434,110. Moreover, the selectivity of the hydration catalyzed by mineral acid is very dependent on the conversion of acrolein. In order to achieve an acceptable selectivity, the hydration is terminated at a low acrolein conversion with the disadvantage of a low space-time yield.
There has been no lack of attempts to reduce the disadvantages of the method evaluated above. For example, carboxylic acid esters of 3-hydroxypropionaldehyde are obtained by means of the attachment of lower carboxylic acids to acrolein by means of heating (U.S. Pat. No. 2,638,479) or in the presence of a basic ion exchange resin, which carboxylic acid esters can be hydrogenated to the corresponding esters of 1,3-propanediol. Disadvantages in these methods are the additionally necessary steps for the saponification of the ester and for the recycling of the carboxylic acid as well as the undesired formation of n-propanol and its carboxylic acid esters during the hydrogenation (DE-OS 20 57 399). The hydration of acrolein with carbon dioxide as catalyst is also known; however, this method requires long reaction times--see DE-OS 19 05 823.
It has been determined that the hydration of acrolein can be carried out using e.g. phosphoric acid or dihydrogen phosphates as catalyst but that problems occur in the subsequent hydrogenation if the hydroxypropionaldehyde solution obtained in this manner is hydrogenated without immediate separation of the hydration catalyst.
If nickel hydrogenation catalysts, which are actually very effective, are used, the catalyst is more rapidly deactivated upon repeated use of this catalyst than if a reaction mixture free of acid and salt is hydrogenated. This results in an elevated consumption of catalyst. In addition, the presence of the hydration catalyst during the workup by distillation causes product losses due to decomposition and, in the case of a preceding neutralization, in cloggings and encrustations in the system. Also, the elimination of distillation residues is more difficult given a content of inorganic salts and thus more expensive than is the case without these salts.
The indicated problems can be partially circumvented if the hydration catalyst is removed from the reaction mixture by ion exchangers before the hydrogenation or if 3-hydroxypropionaldehyde is separated by extraction from the reaction mixture and is then hydrogenated. However, both alternative measures for reducing the consumption of expensive hydrogenation catalyst necessitate additional apparatuses, and result in a greater expenditure of energy and in waste water problems and thus increase the production cost for 1,3-propanediol.
According to the method of U.S. Pat. No. 3,536,763, the hydration of acrolein is carried out at 40.degree. to 120.degree. C. in the presence of weakly acidic cation exchange resins whose functional groups are only carboxyl groups. Preferably, 0.1 to 5% of the functional groups should be present in the form of an alkali-, alkaline-earth- or earth-metal carboxylate. The yields of 3-hydroxypropionaldehyde are indicated at approximately 80% and the yields from the acrolein conversion in the range of 25 to 65% should be practically not dependent. This document also describes the known hydrogenation of 3-hydroxypropionaldehyde to 1,3-propanediol.
During the repetition of the method of U.S. Pat. No. 3,536,763, the catalytic activity of the ion exchange resins with carboxyl groups was able to be corroborated; however, the degree of the activity made the use of these ion exchangers in industrial systems appear to be unsuitable. It turned out that these catalysts require rather high temperatures and rather long reaction times, which runs contrary to the desired high space-time yield and high selectivity.